In two molecules, when the type of an isotope constituting one of the molecules is different from the type of an isotope constituting the other molecule, the peak position of a vibration absorption spectrum in the infrared region for one of the molecules is slightly different from the peak position of a visible absorption spectrum in the infrared region for the other molecule. This difference is called “isotope shift.” In isotope separation using an infrared laser, molecules including a particular atom as a target for isotope separation are irradiated with a strong infrared laser beam to cause multiphoton dissociation of only the molecule comprising the particular isotope by taking advantage of the isotope shift, whereby the dissociation product or the residual molecule is enriched with the target isotope. Lasers usable herein include carbon dioxide lasers, carbon monoxide lasers, free electron lasers, semiconductor lasers, solid-state lasers, and any other laser which has an oscillation wavelength near 1 to 100 μm.
The abundance ratio of isotopes of natural silicon is 28Si:29Si:30Si=92.23%:4.67%:3.10%. A technique for laser isotope separation of silicon (Si) is disclosed in Japanese Patent Publication No. 56133/1990. Specifically, this publication proposes a working substance for separating isotopes of Si by laser isotope separation and an isotope separation method using the working substance. This working substance is a fluoromonosilane compound represented by formula SiaXbHc wherein 2≦a≦3, 0≦6≦2a+2, and 2a+2=b+c; and X's, which may be the same or different, represent a halogen atom. Japanese Patent Publication No. 13685/1993 proposes, as a working substance for separating isotopes of Si by laser isotope separation, a fluoromonosilane compound represented by SiFnX4-n, wherein X represents H, Cl, Br, or I and 1≦n≦3, or SiFnR4-n wherein R represents an alkyl group or a halogen derivative thereof and 1≦n≦3, and an isotope separation method using the working substance. In this technique, molecules such as Si2F6 or SiF3Br are used. Further, SiF3H, SiF3Cl, SiF2H2, SiFCl3, SiF3CH3, SiF3CF3, SiF2(CH3)2 and the like are described as examples of target molecules. In particular, Si2F6 is currently used as a material for studies on practical use of Si isotope separation, because activation energy is low and isotopes can be separated with high efficiency.
In the conventional laser isotope separation methods, however, in many cases, radicals are generated in the course of the reaction. The radicals easily induce a secondary reaction which contributes to lowered selectivity for a target isotope and is causative of the formation of a solid component and a polymer component. The formed polymer component poses serious problems associated with a separation apparatus such as a deterioration in transmittance or damage due to the contamination of the inner surface of the reaction vessel with the polymer or the deposition of the polymer on a laser incidence window. To overcome these problems, for example, a method in which a scavenger gas is mixed to capture the generated radicals and a method in which the formed solid, polymer and the like are regassified by a treating agent for removal and recovery (Japanese Patent Laid-Open No. 259373/2001) have been proposed. The use of the scavenger gas, however, is causative of the dissipation of energy and, further, renders the reaction more complicated. On the other hand, the treatment of the formed solid and polymer more or less disadvantageously causes damage to the laser incidence window.